1. Field Of the Invention
This invention relates to electrical devices and methods for making them, particularly electrical devices which are suitable for use as circuit protection devices.
2. Introduction to the Invention
Electrical devices for use in protecting against overvoltage or over-temperature conditions in a circuit are well known. Such circuit protection devices frequently comprise materials which exhibit a positive temperature coefficient of resistance, i.e. PTC behavior, and thus act to shut down a circuit if conditions are unsafe by increasing in resistance by orders of magnitude from a normal, low temperature value. Devices of this type may comprise an inorganic material, e.g. BaTiO.sub.3, or a conductive polymer composition. For any material, the time required for the device to switch into its high temperature, high resistance state, i e to "trip" is a function of the resistivity of the material, the geometry of the device, and the thermal environment. Similarly, the time required for the device to cool down from its high temperature, high resistance state, i.e. to "reset", is also a function of the material, the geometry of the device, and the thermal environment. It is generally preferred that the resistance of the device at 23.degree. C. be as low as possible in order to contribute as little resistance as possible to the circuit during normal, low temperature operation. For most low voltage applications, i.e. 60 volts or less, devices of planar geometry are preferred. Such planar devices comprise a laminar resistive element which is electrically connected to two laminar electrodes. For a material of a given resistivity, planar devices of specified area will have the lowest resistance when the distance between the electrodes, i.e the current path length, is the smallest. Therefore, thin devices are preferred and result in lower resistances, lower materials requirements, and smaller "real estate" requirements for a printed circuit board.
There are problems, however, with thin laminar devices. When the device trips into its high resistance state, heat is generated by I.sup.2 R heating. Because of the relatively small thermal mass of a thin device, it tends to dissipate the heat rapidly and to trip rapidly. Such rapid tripping is not desirable for all applications. For example, when a device is designed to protect a motor used to raise or lower a window, for example in an automobile, the device heats as the motor operates. It is necessary that the window be fully opened or closed before the device heats sufficiently to cause it to trip. Therefore, a relatively long trip time is needed when compared to many conventional applications. One technique to increase the trip time is to increase the size of the device. This, however, is undesirable if the space available for the device is limited. Another technique to increase the trip time is to increase the thermal mass by electrically and physically attaching elements of high thermal mass, e.g. metal terminal plates, to the device. Alternatively, the laminar electrodes themselves may comprise relatively thick, high thermal mass material. Such electrodes are described in European Patent Publication No. 363,746 (Asea Brown Boveri, published Apr. 18, 1990), the disclosure of which is incorporated herein by reference.